So what's the carbon footprint of one of these launches? Did I understand correctly that we can make the fuel by taking CO2 from the atmosphere? That would make the footprint negative, would it? That seems to good to be true.
CO2 extraction for methane production is planned on mars, where the relative ratio of CO2 in the atmosphere is much higher. On Earth, CO2 is only 0.04%, which makes it quite difficult to extract.
The kicker for Earth is that it's a lot easier and cheaper to make CH4 from biological sources (fossil fuels, fermenters, etc), so nobody does it at scale. All the solutions (IIRC) need high temperature and a catalyst, which isn't surprising as we're un-burning rocket fuel. With cheap plentiful solar energy it's more practical; as Mars has no bio-sources of scale it's essential.
I suppose some of it would be lost to the vacuum of space, though it would have to be an impressively large space transport market for that to start making a global difference.
Also weird to think about the long-term implications of Earth's resources leaving Earth permanently/irrecoverably.
Carbon neutral assuming you are creating the CH4 using atmospheric CO2. On Earth I suspect that most CH4 comes from natural gas. On Mars it would be carbon neutral - but that's actually a bad thing, assuming that your goal is to increase the atmospheric density on Mars.
Yes, that was the assumption of the parent question. I expect that initially all the fuel for Earth launches will be sourced from natural gas, but if they start doing regular sub-orbital passenger flights they'd have to convert to renewable energy + atmospheric CO2, otherwise they'd just (partially) undo the work Tesla is doing.
